Antenna element, conically helical, for polarization purity within a broad frequency range

ABSTRACT

An antenna element including a ground plane. A support supports a conical structure. The support is made of a dielectric material. A bottom portion of the support is attached to the ground plane. Four conical radiation means are symmetrically arranged around the support. Each radiation elements includes a helical wire having a top end and a bottom end. The bottom ends of the radiation elements are attached to the ground plane. Four coaxial cables including conductors are provided. One coaxial cable is connected to the top end of each radiation element for providing each radiation element with a microwave signal for emitting two orthogonally polarized radiations. A distribution network divides an incoming signal into four subsignals offset in phase in relation to each other. One of the subsignals is provided to each of the four radiation elements. Adaption elements adapts an output impedance of the distribution network to an input impedance of the radiation elements. The adaption elements include four channels and the conductor of one of the coaxial cables extending therethrough. Each channel extends through a metal block.

FIELD OF THE INVENTION

The present invention relates to antenna elements, typically for use onsatellites.

BACKGROUND OF THE INVENTION

Antenna elements such as those of the present invention are usedparticularly in group antennas for satellites. Such antennas should havea good polarization purity. That is, such antennas should obtain a lowamount of radiation of non-desired polarization and a high amount ofradiation having desired polarization. At the same time there is a needfor broadband such antennas should obtain the antenna will be able toemit and receive microwave signals within a relatively wide frequencyrange. If the frequency range is limited to one or more narrow bands,the polarization purity itself can be improved but only at the sacrificeof the broadband characteristics.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide an antenna element ofthe kind discussed above, which permits both a high polarization purityand broad band properties. The present invention relates to an antennaelement comprising a ground plane and a conical support of a dielectricmaterial. A bottom portion of the conical support is attached to theground plane and supports first to fourth radiation means having theshape of helical wires arranged symmetrically around and carried by thesupport. The radiation means are, at their exterior, lower ends attachedto the ground plane. For transmission, each radiation means is provided,at their upper, interior parts, through an individual coaxial cable withan individual microwave signal, so that two orthogonal polarizationsthat preferably are circular are generated by the emitted radiation.

According to the present invention such an antenna element is primarilycharacterized in that. for transmission, a distribution network isarranged to divide the incoming signal into four subsignals that areoffset in phase in relation to each other. Each signal is provided toone of the first to fourth radiation means mentioned above. Adaptionmeans are arranged to adapt the output impedance of the distributionnetwork to the input impedance of the radiation means, so that it issubstantially independent of the actual microwave frequency used withina relatively wide frequency range.

In an advantageous embodiment of the antenna element according to thepresent invention, the adaption means comprises four separate conductorsthat constitute capacitive loads, which, with their ends are connectedto the upper ends of a corresponding radiation means.

In an alternative embodiment, the adaption means comprises a metal blockconstructed to include four interior channels through which therespective conductor in the coaxial cables extend substantiallycentrally.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following in greater detail withreference to the accompanying schematic drawings in which:

FIG. 1A shows an elevational view, which partially is a sectional view,of an antenna element according to the present invention,

FIG. 1B shows the antenna element of FIG. 1A as seen from above,

FIG. 2A shows an elevational view, which partially is a sectional view,of an adaption means,

FIG. 2B shows the adaption means of FIG. 2A as seen from above,

FIG. 3A shows an elevational view, which partially is a sectional view,of an alternative adaption means,

FIG. 3B shows the adaption means of FIG. 3A as seen from above,

FIG. 4 shows the input impedance Z of the radiation means as a functionof the frequency in GHz for an older antenna element, graph I, and anantenna element according to the invention, graph II.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1A and 1B a ground plane having the shape of a circular metalplate has the reference numeral 1. A conical support 2 of a dielectricmaterial is, with its bottom, portion attached to the ground plane. Thesupport is constructed from two planes arranged orthogonally in relationto each other and carries at its geometric envelope surface first tofourth radiation means having the shape of helical wires 3 to 6 that arearranged symmetrically around the support. Four coaxial cables, twocables thereof having the reference numerals 8,9 being shown in FIG. 1A,extend up through the center of the support. The conductors in thesecoaxial cables, that are referenced 7-10, are, at their top portions,joined to one helical wire 3-6 each. The latter ones are, at theirbottom portions, joined to the ground plane 1. The lobes of the antennascan be varied by changing the conical apex angle of the support and theangular pitch of the helical wires.

In this embodiment of the antenna element according to the presentinvention, adaption means having the shape of four separate conductors11 to 14 are directly connected to, that is, by being soldered to an endof an above mentioned conductor 7-10, before the connection thereof tothe respective radiation means. These separate conductors 11-14 are thusconstituted of short metal wires each having a non-connected connectedend free so that they constitute capacitive loads.

The antenna signal is fed through a distribution network 15, not shownin detail. The signal is divided in four signals having the sameamplitude but having phases distributed at the angular values of 0°,90°, 180°, and 270°. These signals are delivered to the four coaxialcables.

The distribution network, the adaption means and the radiation means arenow so arranged that a high polarization purity is obtained within awide frequency range. If the elevational lobe of the antenna element ismaintained constant and is varied azimuthally, a minimal variation ofthe radiation of the desired polarization, that can be linear orelliptical, in particular circular, is obtained.

It is possible to use the adaption means shown in FIGS. 1A and 1B withinthe frequency range of 2.0 to 2.3GHz, for example. In FIG. 4, forexample a comparison is shown of the input impedance Z of the radiationmeans for an older design of an antenna element of the kind mentioned inthe introduction, by line I, and by line II for an antenna elementaccording to the invention. It is apparent that the impedance isrelatively independent of the frequency of the antenna element accordingto the invention.

An alternative embodiment of the adaption means having the shape of anadaption transformer is shown in FIGS. 2A and 2B. It consists of a metalblock 16 having four interior channels 17, through which the respectiveconductor 18 of the coaxial the cables 8, 9 extend substantiallycentrally, having distance washers of a dielectric material. Thisadaption means is placed at the top of the antenna element, close to theconnection to the radiation means, and is suited for use, for examplewithin the frequency range of 1.2 to 1.6 GHz.

A variant the last mentioned embodiment of the invention shown in FIGS.3A and 3B comprises adaption means that include four metal blocks 19.Each block has an interior channel 20, through which one of the fourconductors 21 in the coaxial cables 8, 9 extends substantiallycentrally. The four metal blocks 19, which are similar to each other,are arranged, as seen in a cross sectional view shown in FIG. 3B, in asquare pattern at some distance from each other.

We claim:
 1. An antenna element, comprising:a ground plane; a support for supporting a conical structure, the support being made of a dielectric material, a bottom portion of the support being attached to the ground plane; four conical radiation means symmetrically arranged around the support, each radiation means comprising a helical wire having a top end and a bottom end, the bottom ends of the radiation means being attached to the ground plane; four coaxial cables including conductors, one coaxial cable being connected to the top end of each radiation means for providing each radiation means with a microwave signal for emitting two orthogonally polarized radiations; a distribution network for dividing an incoming signal into four subsignals offset in phase in relation to each other, one of the subsignals is provided to each of the four radiation means; and adaption means for adapting an output impedance of the distribution network to an input impedance of the radiation means, the adaption means comprising four channels and the conductor of one of the coaxial cables extending therethrough, each channel extending through a metal block.
 2. The antenna element according to claim 1, wherein the adaption means is arranged in the vicinity of the top ends of the radiation means.
 3. The antenna element according to claim 2, wherein the channels of the adaption means all extend through one metal block.
 4. The antenna element according to claim 2, wherein each of the channels of the adaption means extends through a separate metal block.
 5. The antenna element according to claim 4, further comprising:distance washers for maintaining the conductors of the coaxial cables centrally located in the channels of the adaption means.
 6. The antenna element according to claim 5, wherein the distance washers comprise dielectric material.
 7. The antenna element according to claim 5, wherein the channels of the adaption means all extend through one metal block.
 8. The antenna element according to claim 3, wherein each of the channels of the adaption means extends through a separate metal block.
 9. The antennal element according to claim 1, wherein the microwave signal is distributed to each coaxial cable phase shifted with angles of 0°, 90°, 180°, and 270°.
 10. The antenna element according to claim 9, wherein the channels of the adaption means all extend through one metal block.
 11. The antenna element according to claim 9, wherein each of the channels of the adaption means extends through a separate metal block.
 12. The antenna element according to claim 1, wherein the channels of the adaption means all extend through one metal block.
 13. The antenna element according to claim 1, wherein each of the channels of the adaption means extends through a separate metal block.
 14. The antenna element according to claim 1, wherein the channels of the adaption means are arranged in a square pattern.
 15. The antenna element according to claim 1, wherein the channels are all parallel to each other. 